In the open front part of the press, where the pressing work is carried out, there is a horizontal table 1 on which a press tool 2 is arranged and at the opening 3, which the frame 4 forms above the table, the press has a slide 5 which is guided in the vertical direction in such a way that it can be displaced towards and from the press table. In its displacement downwards towards the table 1, the slide transmits the force P, necessary for the pressing work, to the tool and the workpiece which is inserted therein and is to be reshaped.
The force from the press slide 5 has the result that the frame 4 upon each such working stroke undergoes a small elastic deflection, followed by a spring-back movement when the slide turns and moves upwards. At the same time as the frame 4 is deflected, the opening 3 is widened slightly upwards, which thus takes place just at that moment in the cyclical operation when the pressing work is being performed. This unavoidable situation in C-frame presses is well known to those skilled in the art. It is usually said that the press is "yawning".
At the moment when the press "yawns", the slide 5 does not move exactly linearly to and from its turning-position, but instead, as a consequence of the deflection and spring-back of the frame 4, the direction of the slide relative to the press table 1 varies slightly in such a way that a small angular movement outwards and inwards can be observed. In FIG. 2, which shows diagrammatically the movement at the pressing moment, this angular variation is designated by .alpha., the line 6 is the ideal direction of a central plane in the slide 5, in which plane the force P acts and along which plane the slide is intended to move at right angles to the press table 1, whereas the line 7 is the inclined position (shown exaggerated) which the slide plane 6 assumes at the moment when the force P has reached its maximum, that is to say when the "yawning" is at its greatest.
When the slide movement is transmitted in a conventional manner to the tool 2 and the workpiece in the latter, the inclination .alpha. has the result that the useful or effective part of the working stroke, that is to say the slide movement downwards towards the press table 1 corresponding to the reshaping of the workpiece, is not equal at different points along a transverse horizontal plane, for example the plane 8 in FIG. 2. As the figure shows (again slightly exaggerated), the effective working stroke is less at the outer (in the figure right-hand) side 9 of the slide, the tool and the workpiece than at the central plane 6. The working stroke or the press depth increases successively along the broken line 10 from the said side to the inner opposite side 11 where the generated press depth S.sub.2 is greatest. The full line 12 is intended to represent the desired turning-position which should guarantee a constant press depth amounting to S.sub.2 over the whole plane 8.
The variation in press depth, which is represented in FIG. 2 by lines 10 and 12 and which occurs in all work on a C-frame press according to previously known technology, is the same or essentially the same in each plane which is perpendicular to the central plane 6, that is to say parallel to the abovementioned lines. There is thus no difference or only a slight difference in the press depth at various points along the central plane or another plane parallel to it. However, in the case of a frame which has a design other than that shown in principle here, or in the case of a certain tool construction, it is possible for the maximum difference S.sub.2 -S.sub.1 in the press depth to occur in a direction which is not perpendicular but instead oblique relative to the central plane 6, in which respect each arbitrary line which connects points of the same press depth correspondingly crosses obliquely over the press table.
It will be understood from the above that the inclination of the slide, which occurs at the pressing moment, results in considerable shaping errors in the pressed components. The shaping errors are often unacceptably great, in particular in the case of components which take up a large part of the size of the press table in the transverse direction, that is to say in the direction 9-11. An example of such components is blades forming part of fan wheels which are to have an arched cross-profile, identical along the whole blade length. In FIG. 3 the full lines indicate the desired cross-section of such a blade. Since, for industrial engineering reasons, the longitudinal direction of the workpiece introduced into the press tool coincides with the direction of the line 10, the profile depth in the pressed component decreases in the direction towards the outside 9 of the tool to the same extent as the working stroke decreases along this line. The cross-section of the finished blade can, at the outer (right-hand) end, have the appearance which is shown diagrammatically in the figure by broken lines. An error of this type can lead to problems when assembling the fan wheel and can also mean that the performance of the fan is poorer than when the blades have a correct, constant cross-section.
Another problem caused by the inclination is illustrated in FIG. 4, in which 13 designates a die tool comprising a die 14 which, upon pressing of a hole in a sheet blank 15, moves downwards into an opening 16, adapted exactly to the die, in the counter-die 17 of the tool.
If in this case too there is an error in the direction of the die 14, as shown by inclination angle .alpha., the outer (in the figure right-hand) edge of the die, upon its movement downwards into the counter-die, strikes against the latter in the upper part of the opening 16 where it meets the counter-die at 18, with the consequence that the tool here undergoes significant wear, which considerably shortens its lifetime. The problems which result from inclination of the slide can only be partially counteracted by using a larger press with a stronger frame than is required for the pressing work, and this is economically disadvantageous. The alternative of going over to another type of press, so called 2- or 4-column presses, where no "yawning" occurs, is even more disadvantageous. On the one hand, the initial outlay for such presses is higher than for the C-frame press, and, on the other hand, the pressing work cannot be carried out at the same high rate on account of the fact that the space is limited by the columns.